To protect each inverter by detecting the open phase in the inverter, by stopping the operation of the inverter when the value of the input voltage of the inverter is positive and the output voltage from the inverter is zero.

A current detector 18 detects the input currents I1 of inverters 6 and 7, and a discriminating section 19 inputs the currents I1) detected by means of the detector 18 and, when the values of the currents I1 are positive (I1>0), outputs a signal '1'. Another discriminating section 20a inputs the phase current I0a detected by means of another current detector 16 and, when the current I0a is 0 A, outputs a signal '1'. Therefore, when, for example, an open phase occurs in the inverter 6, the output of the discriminating section 20a becomes the signal '1', because the phase current I0a detected by means of the detector 16 becomes 0 A. In this case, the operations of the inverters 6 and 7 are stopped. Therefore, the inverter 7 in which no open phase occurs is prevented form being overloaded.